The differences between controls and behaviors are mostly semantic. Both are components associated with DOM elements in the page, and they offer a similar set of features. Behaviors enhance DOM elements without changing the base func- tionality they provide. If you associate a behavior with a text box element, the text field continues to accept the user’s text input. But you can use the behavior to add client functionality to the text box and, for example, upgrade it to an auto-com- plete text box.
The chief purpose of client controls is creating element wrappers. For example, you can create a TextBox control that wraps an input element of type text. You can create a Label control that wraps a span element, and so on. This is similar to what happens with the ASP.NET TextBox and Label server controls. The differ- ence is that they wrap DOM elements on the server side rather than on the client side. An element wrapper can be useful to enhance the way you program against a DOM element. For example, you can use controls as a way to program against DOM elements using declarative code.
A fundamental difference between behaviors and controls is that a DOM ele- ment can have multiple behaviors, but it can be associated with one and only one control. For this reason, behaviors are best suited to add client capabilities to a DOM element in an incremental way. On the other hand, a control is supposed to provide the whole client functionality to its associated element.